In order to maximize the cariostatic effects of fluoride while minimizing fluorosis, one must understand the "critical period" for the manifestation of fluorosis. Clinical trials to test for this "critical period" cannot be conducted today because of ethical concerns, but opportunistic epidemiology provided a quasi-experimental design to study the effects of cessation of water fluoridation. During an 11-month time span (September 6, 1990 to mid-August 1991), the public water supply of Durham, NC, was not fluoridated as it normally had been. The aim of the original study (1995-1998) was to assess the impact of this 11-month break in water fluoridation on the development of dental caries and fluorosis. The research design was a follow-up cohort comparison with a nested case-control study, which analyzed clinical and interview data from 6 cohorts of school children in the elementary grades of Durham Public Schools. The original 6 cohorts (n=238) were established to relate the interruption in fluoridation to the ages of the children at the time it occurred. Results showed no caries effects, but children who were not exposed to fluoridated water between 1-3 years of age had much less fluorosis than did children who were exposed. The aim of the present continuation study is to test the hypothesis that children born after water fluoridation was fully restored have the same prevalence and severity of fluorosis as did children from the original study who were fully exposed to fluoridated water at 1-3 years of age. The research design is identical to the original work, but with 2 additional cohorts of schoolchildren in Durham Public Schools born at the time or after fluoridation was restored. This continuation study plans to examine the 2 additional cohorts, one in the first year and one in the second year, to standardize age at which data are recorded. Interviews for demographic information and fluoride histories will be carried out by a trained interviewer. Comparison of results between different cohorts will permit firm conclusions on ages at which fluoride exposure has its greatest impact on fluorosis, and the case-control analysis will provide quantification of the risk. Results will permit a clearer view of the impact of water fluoridation in modern conditions of multiple fluoride exposure. If the fluorosis experience of the cohorts born after water fluoridation was restored is not significantly different from earlier cohorts who were fully exposed at ages 1-3, then the concept of a "critical period" for fluorosis will be confirmed.